Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a duplexer (diplexer, triplexer, quadplexer, quintplexer, etc.) for example, connected between an antenna (there could be several antennas like for MIMO) and a transceiver for filtering received and transmitted signals.
Various types of filters use mechanical resonators, such as surface acoustic wave (SAW) resonators. The resonators convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals.
SAW resonators can be provided over high-resistivity, monocrystalline silicon substrates, so that radio-frequency (RF) losses due to currents in the substrate generated by electric fields from the electrodes are mitigated. However, in spite of the use of a high-resistivity, undoped monocrystalline silicon substrate, an inversion channel can be formed due to charges in other layers in the SAW structure. Carriers in the substrate, in turn, can be injected into the inversion layer. Spurious currents can result from these charges due to the electric fields generated by the electrodes. Thus, in known SAW structures, RF losses due to spurious currents can remain.
What is needed, therefore, is a SAW resonator device that overcomes at least the shortcomings of known SAW resonators described above.